1. Field of the Invention
The present invention relates to an in-plane switching mode liquid crystal display device, and more particularly, to an in-plane switching mode liquid crystal display device and a fabrication method thereof which can improve image quality by preventing VAC (Viewing Angle Cross Talk) deficiency.
2. Description of the Related Art
Liquid crystal displays are typically used as flat panel display devices which have low power consumption and provide high picture quality. A liquid crystal display device is formed by attaching face to face a thin film transistor array substrate and a color filter substrate with a uniform interval therebetween, and disposing a liquid crystal layer between the thin film transistor array substrate and the color filter substrate. Pixel regions are arranged on the thin film transistor array substrate in a matrix manner. A thin film transistor, a pixel electrode and a capacitor are formed within the pixel region. A common electrode and the pixel electrode-apply an electric field to the liquid crystal layer. An RGB color filter and a black matrix are formed on the color filter substrate.
Meanwhile, an alignment film is formed at surfaces of the thin film transistor array substrate and the color filter substrate facing each other and is rubbed to orient the liquid crystal material in a specified direction. When an electric field is applied between the pixel electrode and the common electrode, the liquid crystal material rotates due to dielectric anisotropy. As a result, light is transmitted or blocked by pixels to display the image. However, such a twisted nematic mode liquid crystal display device has a narrow viewing angle.
Accordingly, an in-plane switching mode LCD has been recently introduced to improve the narrow viewing angle by aligning liquid crystal molecules in a substantially horizontal direction with respect to the substrate.
FIGS. 1A and 1B schematically illustrates pixels of a typical in-plane switching mode liquid crystal display device. FIG. 1A is a plane view and FIG. 1B is a cross-sectional view taken along line I-I′ of FIG. 1A. As shown therein, gate lines 1 and data lines 3 are arranged horizontally and vertically on a first transparent substrate 10 to define the pixel regions. Although in an actual liquid crystal display device, there are N gate lines 1 and M data lines 3 crossing each other to create N×M pixels, only two pixels are shown in the drawing for explanatory purposes.
A thin film transistor 9 is disposed at a crossing of the gate line 1 and the data line 3. The thin film transistor 9 includes a gate electrode 1a, a semiconductor layer 5 and source/drain electrodes 2a and 2b. The gate electrode 1a is connected to the gate line 1. The source/drain electrodes 2a and 2b are connected to the data line 3 and a pixel electrode 7, respectively. A gate insulation layer 8 is formed on the entire substrate 10.
A common line 4 is arranged parallel to the gate line 1 in the pixel region. A pair-of electrodes, which are the common electrode 6 and the pixel electrode 7, are arranged parallel to the data line 3 for switching liquid crystal molecules. The common electrode 6 is simultaneously formed with the gate line 1 and is connected to the common line 4. The pixel electrode 7 is simultaneously formed with the source/drain electrodes 2a and 2b and is connected to the drain electrode 2b. A passivation layer 11 is formed on the entire surface of the substrate 10 including the source/drain electrodes 2a and 2b. In addition, the pixel electrode line 14 formed to overlap the common line 4 forms a storage capacitor (Cst) with the gate insulation layer 8 interposed therebetween.
In addition, a black matrix 21 and a color filter 23 are formed on a second substrate 20, on which an overcoat layer (not shown) may be formed for flattening the color filter 23. The black matrix 21 prevents light leakage where the thin film transistor 9, the gate line 1 and the data line 3 are located. The color filter 23 provides color display capabilities to the liquid crystal display device. In addition, alignment films 12a and 12b are applied at the surfaces of the first and second substrates 10 and 20 facing each other. The alignment films 12a and 12b determine an initial alignment direction of the liquid crystal.
Also, a liquid crystal layer 13 is formed between the first and second substrates 10 and 20. The liquid crystal layer 13 controls the light transmittance by a voltage applied between the common electrode 6 and the pixel electrode 7.
The conventional in-plane switching mode liquid crystal display device having such a construction can improve a viewing angle because the common electrode 6 and the pixel electrode 7 are arranged on the same substrate and generate an in-plane electric field.
On the other hand, light leakage may occur at a specific viewing angle when misalignment between the common electrode 6 and the data line 3 of the first substrate 10 or misalignment between the first substrate 10 and the second substrate 20 occurs.
FIGS. 2 and 3 are sectional views of a liquid crystal display device to illustrate the problems in the related art, wherein an alignment film is omitted. As shown in FIG. 2, when the misalignment between the common electrode 6 and the data line 3 occurs and therefore the data line 3 is shifted to the left, the data line 3 becomes relatively far from the common electrode 6 formed on a right pixel. Accordingly, light leakage (indicated by arrow in the drawing) through a gap between the data line 3 and the common electrode 6 of the right pixel occurs at a specific range of viewing angles. That is, when the viewer is in front of the LCD device, the light leakage can be blocked by the black matrix 21 formed on the second substrate 20. However, when the viewer faces the LCD device within the specific range of viewing angles, the light leakage will be seen in that specific area where light is not blocked by the black matrix 21.
Accordingly, an undesired line is observed on a screen because of the light leakage between the data line 3 and the common electrode 6 of the right pixel at the specific range of viewing angles. A defect displayed on the screen because of the light leakage occurred at the specific range of viewing angles is referred to as VAC (Viewing Angle Cross Talk) deficiency.
Also, as shown in FIG. 3, when the misalignment between the first substrate 10 and the second substrate 20 occurs in the process of attaching the first substrate 10 and the second substrate 20 to each other, the black matrix 21 formed on the first substrate 10 inclines toward a left pixel. Therefore, light leakage occurs between the data line 3 and the common electrode 6 formed on the right side of the data line 3. In particular, the VAC deficiency is observed only at the specific range of viewing angles.